1. Field of the Invention
The present invention relates to lightweight, compact and high performance solenoid valves suitable for use the transmission of passenger and other vehicles.
2. Description of the Prior Art
Recently, a hydraulically controlled electromagnetic valve has been increasingly used in motor vehicles. Such an electromagnetic valve is essential for various automatic transmissions, and particularly necessary for lock-up control by a four-speed automatic transmission with an autodrive unit and pattern changes by an electronically controlled automatic transmission. For various clutch operations, today's electromagnetic valve acts not only as an on-off valve, but also as a pressure control valve with PMW (pulse width modulation) control. Thus, there is a demand for an electromagnetic valve which is highly reliable and more compact and lightweight than a conventional electromagnetic valve.
Such a conventional solenoid valve typically includes a poppet wherein movable parts are subject to hydraulic pressure. There has recently been proposed a solenoid valve of the type wherein movable parts are in no way subject to hydraulic pressure, for example, as shown in Japanese patent publication No. 61/25953. This type of solenoid valve is designed as a compact and high performance valve. As shown in FIG. 15, this solenoid valve is an electromagnetic valve which includes a body, and a poppet slidably fit within the body and moved to and from a valve seat to open and close a fluid passage. A guide rod is fixed to the body. The hollow poppet is fit around the upper end of the guide rod in an oil-tight manner and slidable therealong. The valve seat is fixed to the bottom of the guide rod. The fluid passage is communicated with the poppet through the interior of the guide rod. The effective diameter of the seal area between the poppet and the valve seat is identical to that of the seal area between the poppet and guide rod. However, this arrangement suffers from the following disadvantages.
First, a coil spring is used to urge or press the poppet. The area where the poppet is seated on the valve seat needs to be large in terms of its structure and also for the purpose of preventing the loss of flow. To this end, it is necessary to provide not only a stronger solenoid, but also a stronger spring for retuning the poppet to its original position when the solenoid is deenergized. A disadvantage with such stronger solenoid and spring is that the threshold value of the working pressure becomes low. Also, in a pressure balance-type valve, the valve can not be opened if the pressure within a circuit reaches an extraordinary value. Therefore, a relief valve must be added to the valve assembly.
Secondly, a disadvantage with such a poppet-type electromagnetic valve is the tendency to generate noise during high-speed operation. When the valve is repreatedly opened and closed at high speed, particularly under flow control by pulse signals, noise is continuously generated thereby causing a serious problem. It is assumed that such noise occurs when two metal parts contact and when hydraulic oil flows at high speed. To this end, there is proposed a poppet, a valve seat or a spacer in the solenoid made of nonmetallic material having an acoustic property. However, such nonmetallic material is not durable and may be used for special purposes, only. To prevent noise due to jet flow of the oil, the solenoid valve may be fully submerged within the oil. However, this may result in an increase in the back pressure and thus resistance to flow of the oil discharged from the valve in which case the valve can not be repeatedly operated at high speeds.
Thirdly, at the beginning of the stroke of an electromagnet, as a drive source for an electromagnetic valve, or when a stator is spaced a long distance from an armature, the magnetic attraction is small. At the end of the stroke of the electromagnet, or when the space between the stator and the armature becomes smaller, magnetic attraction tends to suddenly become greater. In a proportional control valve, it is desirable that the magnetic attraction be in proportion to the input current despite the stroke of the electromagnet. An electromagnet for a proportional control valve used in a hydraulic system may be designed for this effect. One example of a direct current electromagnet is shown in Japanese utility model publication No. 52/56449. As shown in FIG. 16, this direct current electromagnet includes an armature having a converging or tapered end, and a stator having a recess. This recess has a divergent inner surface. The stator has a converging or tapered end, so that the armature can be rapidly attracted to the stator at the beginning. This prevents a sudden increase in the magnetic attraction when a space y between the armature and the stator becomes small.
Such a direct current electromagnet, when used as a compact and high speed proportional control valve for a vehicular transmission, can not provide sufficient magnetic attraction at the beginning of the stroke as shown by broken lines i.sub.1, i.sub.2 and i.sub.3 in FIG. 12. As a result, the magnetic attraction suddenly increases when the space y becomes smaller.